Lightwave networks are increasingly being used to rapidly transfer information around the world. Lightwave networks include a number of stations, or nodes, that are interconnected by waveguides, typically optical fibers.
Inside an optical fiber, pulses of light are transferred over long distances with minimal loss. The conventional sources of the light pulses in lightwave networks are laser diodes (LD). At each end of the optical fibers in lightwave networks are various hardware devices, such as switches, amplifiers, multiplexers, and demultiplexers, that are critical to the function of lightwave networks.
Fiber Bragg gratings (FBGs) are important building blocks in a variety of lightwave network devices. An FBG acts to reflect light energy having a certain wavelength back in the direction from which the light originated. When a series of different wavelengths is present in a waveguide, FBGs can be used as filters to isolate light energy having a particular wavelength.
An FBG establishes a periodic change in refractive index along a core of an optical waveguide, typically an optical fiber. At each period, a portion of the optical wave is reflected, inducing interference in a constructive manner. The strength of the change in refractive index along with the grating period and the length of the FBG are factors that determine the range of wavelengths that will be reflected, as well as the efficiency of reflection.
The filtering properties and versatility of FBGs have led to the use of FBGs in such devices as wavelength-stabilized lasers, fiber lasers, remotely pumped amplifiers, Raman amplifiers, wavelength converters, passive optical networks, wavelength division multiplexers, demultiplexers, add/drop multiplexers, dispersion compensators, and gain equalizers. An add or drop multiplexer allows a specific signal or channel to be added to or dropped from a group of channels. Adding and/or dropping a specific channel is important because optical data is often transmitted in a multiplexed condition, whereby multiple channels of varying wavelengths are sent simultaneously over a single optical fiber. Using an add and/or drop multiplexer allows a particular channel to be singled out from a group of, for example, eight, sixteen, or possibly sixty-four channels. A dispersion compensator compensates for the time delay that develops when an optical pulse spreads due to its different wavelengths traveling at different speeds.
An example of a conventional add/drop multiplexer 300 is shown in FIG. 1. A first three-port circulator 302 is connected to an input fiber 304, a drop fiber 306 and a central fiber 308. The central fiber includes four FBGs 310, 312, 314 and 316 and is connected to a second three-port circulator 318 to function as an input. An add fiber 320 and an output fiber 322 are also connected to the second circulator 318. In a drop operation, one of a group of optical carriers that propagate from the input fiber 304 to the central fiber 308 is reflected back to the circulator 302 by an appropriately constructed FBG and is directed to the drop fiber 306. In an add operation, an optical carrier is introduced to the central fiber from the add fiber 320 via the second circulator 318, but is reflected back to the second circulator for output via the output fiber 322. The conventional add/drop module does not have the ability to adjust to fluctuations in optical carrier wavelength. The FBGs are manufactured to the predetermined wavelengths of the target optical carriers and no dynamic adjustment is attempted.
Many examples of the use of FBGs in add and/or drop multiplexers exist. U.S. Pat. No. 5,555,118 to Huber discloses a method for removing and inserting optical carriers in a wave-division multiplexed (WDM) optical communication system, and U.S. Pat. No. 5,600,473, also to Huber, discloses optical amplifier systems with add/drop multiplexing. The Huber patents disclose the use of FBGs in add and drop multiplexers of the type used to manipulate optical cable TV signals in cable TV networks but do not disclose the ability to adjust the FBGs in response to fluctuations in the wavelength of an optical carrier.
European Pat. No. EPO 0730172 A1 to Chawki also discloses an optical add/drop multiplexer using optical circulators and a photo-induced Bragg grating. The disclosure acts in much the same way as the Huber systems to add and/or drop at least one optical signal with a determined wavelength from a group of signals. Each FBG is set to a predetermined wavelength and is able to be tuned in a first state in which the FBG reflects the signal with the predetermined wavelength, thereby transmitting the signals with other wavelengths. One or more FBGs can also be set in a second state, in which the adjusted FBGs transmit all of the signals. Chawki's add/drop system has two circulators and four FBGs, such as the one shown in FIG. 1. Signals are dropped through the left circulator and added through the right circulator.
Lastly, an article by C. R. Giles, "Lightwave Applications of Fiber Bragg Gratings," is published in the Journal of Lightwave Technology, Volume 15, No. 8, August 1997. In the article, a four channel FBG add/drop multiplexer and an FBG dispersion compensator are disclosed. Neither of the two devices provides a system that allows the FBGs to be adjusted in response to fluctuations in the wavelength of a target optical carrier.
The delivery of lightwave data in all of the above-described documents is similar. To deliver lightwave data in an optical network, light is pulsed through a waveguide. The light is typically sent at a known wavelength, and digital data is modulated onto the carrier wavelength. The carrier wavelength, known as the optical carrier, is most effective when the carrier wavelength is fixed throughout transmission. Maintaining a constant carrier wavelength is especially important to devices utilizing FBGs. As described above, FBGs are wavelength-dependent and are typically fabricated to operate on a particular wavelength or within a narrow bandwidth of wavelengths.
Unfortunately, the conventional sources of light, such as laser diodes, are not able to generate a stable optical carrier with the wavelength locked within the tolerances presently desired. For example, FBGs can effectively filter out a signal with a bandwidth of 0.2 nm at a single wavelength of 1550 nm. Therefore, if the wavelength of the optical carrier varies outside the bandwidth of the FBG, the filtering efficiency of the FBG is greatly reduced. This is true even for wavelength tunable FBGs that have the ability to effect different optical carrier wavelengths. If the filtering efficiency of an FBG is reduced, the effectiveness of optical devices such as add/drop multiplexers and dispersion compensators declines. An unstable optical carrier may allow the optical carrier to inadvertently propagate through a properly tuned FBG. Conversely, an unstable optical carrier may cause the optical carrier to be inadvertently reflected by an FBG.
What is needed is a method and system that allows the dynamic adjustment of a fiber Bragg grating in response to a varying optical carrier wavelength so that the optical carrier can be effectively manipulated in optical devices such as add/drop modules and dispersion compensators.